Interpretive Summary: Melon is a horticultural crop species in a relatively large plant family (the Cucurbitaceae) that has worldwide economic importance. Several morphologically distinct wild and cultivated types exist having diverse geographical origins. Cultivated melon market types have been variously classified (taxonomically and horticulturally) based primarily on fruit morphology into six groups. The most economically important are Groups Cantaloupensis and Inodorus, which house muskmelon (cantalope and honeydew (casaba and honeydew) market types, respectively. Both are economically important in the U.S. as fresh market and shipping types. The fruit morphology within these groups is diverse, and depends on geographic origin (i.e., adaptation to environmental factors), cultural traditions, culinary attributes, and market characteristics and requirements. Commercial breeders and traditional farmers (home gardeners and hobbists) consider these factors during germplasm enhancement, which has led to a diverse array of market classes within a horticultural group (e.g., Charentais, Galia, Shipping, Ananas, and Ogen are Group Cantalupensis market types, and Honeydew and Casaba (White, Yellow, Tendral, Green, Piel de Sapo, and Rochet) are Group Inodorus market types. Europe is characterized by extraordinary geographic, agricultural, and cultural diversity, and Spain is a secondary centre for melon diversity, where commercial Group Cantalupensis and Inodorus melon production constitutes about 4% of the world's production. Spanish landraces (horticulturally unimproved populations) are of interest to the U.S. market (especially honeydews) since they are genetically diverse and very different from U.S. market types. The Madrid provenance, has had a rich agricultural history due to its historically strategic geographic and political location, where commodities have been traded nationally for hundreds of years. Thus, a study was designed to determine the genetic diversity of Madrid provenance landraces to determine their utility for plant improvement of U.S. melons using various biotechnologies. It was determined that the landraces examined are dramatically different from U.S. melons and that they hold promise for enhancing U.S. melon market types. Such improvements will provide greater managerial flexibility for U.S. producers and strengthen their ability to compete in global markets.

Technical Abstract:
Melon (Cucumis melo L.) landraces of the Madrid provenance, Spain, have received national distinction for their high fruit quality and sensorial attributes. More specifically, a unique array of Group Inodorus landraces have been continuously cultivated and conserved by farmers in the municipality of Villaconejos since the 19th century. Their genetic relationships to other Group Inodorus and Flexuous melon market classes are not known, and, thus, a study was designed to determine their genetic relationships using 52 simple sequence repeat (SSR) markers, and then make genetic comparisons between these accessions and a previously published "Standard Reference Germplasm Array" (RA) containing Group Inodorus (15 Spanish and one USA), Flexuosus (1), and Cantalupensis (2 USA) melon accessions. This subset consisted of 15 Spanish Group Inodorus landraces that circumscribed the genetic variation of major Spanish melon market classes (Groups Inodorus and Flexuosus), and USA commercial varieties (Groups Cantalupensis and Inodorus). Based on genetic distances, Villaconejos (Madrid) genotypes differed substantially from RA subset accessions, thus defining their genetic uniqueness. Principal component analysis (PCA) partitioned the accessions examined into four distinct groups revealing that Villaconejos black epidermis melons (landraces 'Largo", 'Largo Negro Escrito', and 'Puchero') were distinctly different from all other accessions examined, as cluster analysis separated Rochet market type Villaconejos' accessions (landraces 'Mochuelo', 'Mochuelo Tradicional', and 'Melon de Villaconejos') from RA of the same market type. Genetic assessment of principal Spanish market classes revealed comparatively low intra-market heterogeneity in Piel de Sapo type accessions and high heterogeneity in Black and Yellow market type accessions. While a relatively high level of genetic introgression was detected between Yellow and Green market types, black epidermis market types were genetically unique. Given the uniqueness and high genetic diversity resident in Villaconejos landraces, this germplasm pool should be considered as a genetic source for broadening the comparatively narrow genetic base of Group Cantalupenis and Inodorus melon market types, especially standard commercial Spanish Group Inodorus market types (e.g., Piel de Sapo, Rochet, and Canari).